Question

Q4. State whether the following sentences are true or false, and give the reason behind your answer. 1. Two equally rate three-phase induction and permanent magnet synchronous motors with unknown name plate data. At no load one of them draws a current of 40 A, and he second motor draws 50A. It is stated that the 50 A belongs for the induction motor. 2. Star delta starter is preferred for an induction motor coupled with a high mechanical load. 3. Synchronous motor started with a squirrel cage winding is suffering from a large speed oscillation with a sudden change in load. 4. The three-phase induction motor has a higher efficiency from the single- phase induction motor. However, the three-phase induction motor has an oscillating torque. 5. The parallel connected alternator improves the system reliability but, increase the power generated cost. (25M)

Answer 1

1. (true)Since induction motor has high air gap compared to synchronous motor, induction motor draws high no-load current to setup flux. Hence 50A belongs to induction motor

2. (True) In start delta starter starting current is reduced to 1/3 rd of line current and produces higher torque per ampere of line current.

3.(False) The presence of squirrel cage winding increases the damping nature so that the oscillations die out quickly.

4. Three phase induction motor has high efficiency due to high operating power factor than a single phase induction motor. However ,three phase induction motor produces rotational torque due to it has rotational magnetic field

5. Parallel connected generator increases reliability and reduces cost of generation. Because it reduces the spare cost to half as already standby generators are used in parallel.